The present invention relates to an f.theta. lens system for use in a light scanning device.
Light scanning devices are known as devices for writing or reading information by scanning a light beam. One such light scanning device includes a rotating polygonal mirror having a reflecting surface positioned near a focused linear image formed by a light beam emitted from a light source. The rotating polygonal mirror deflects the light beam at a constant angular velocity. The deflected light beam is focused as a light spot on a surface by a focusing lens system to scan the surface.
The light scanning device employing a rotating polygonal mirror has the problem of irregular deflecting surfaces which do not lie parallel to each other. Since the deflected light beam has a constant angular velocity, the surface should be scanned at a constant speed. An f.theta. lens system is a lens system designed optically to achieve scanning the surface at a constant speed.
One known scheme to correct such irregular reflecting surfaces employs an anamorphic lens system disposed between the rotating polygonal mirror and the surface being scanned, with the reflecting position of the rotating polygonal mirror and the surface being scanned being positioned in conjugate relation with respect to an auxiliary scanning direction of the surface being scanned.
Japanese Laid-Open Patent Publication No. 59-147316 discloses an anamorphic f.theta. lens system for effecting constant-speed scanning and solving the problem of irregular reflecting surfaces. The disclosed lens system cannot however produce a high-density light spot diameter because the curvature of field thereof is not sufficiently corrected in both main and auxiliary scanning directions.